Nodemgr's ignore_drivers variable is exposed as a module load parameter(therefore also as a sysfs attribute below /sys/module) and additionallyas an attribute below /sys/bus/ieee1394. Since the latter is writable,make the former writable too.

Note, the bus's attribute ignore_drivers is only relevant to newly addedunits, not to present or suspended or resuming units. Those have theirown attribute ignore_driver.